Method of manufacturing target electrodes



June 14, 1949. A. ROSE 2,473,220

METHOD OF MANUFACTURING TARGET ELECTRODES Filed Aug. 16, 1941 VAR INVENTOR ALBERT 1 11:1315v Patented June 14, 1949 METHOD OF MANUFACTURING TARGET ELECTRODES Albert Rose, East Orange, N. 3., assignor to Radio Corporation of America, a corporation of Delaware Application August 16, 1941, Serial No. 407,131

11 Claims. 1

My present invention relates to television transmitting tubes and electrode structure and more particularly to methods of manufacturing electrode structures suitable for use in tubes of the low velocity electron beam scanning and electron image types. This application is a continuation in part of my copending original application Serial No. 357,543, filed September 20, 1940, for which application Serial No. 631,441 was filed November 28, 1945, as a continuation, after which application Serial No. 357,543 was abandoned.

In television pickup tubes utilizing high velocity electron beam scanning with optical or electron image projection on a surface other than the surface scanned by the high velocity beam it has been proposed to utilize double-sided mosaic-type targets, as disclosed in British Patent 442,666. Such target electrodes are exceedingly diificult to construct, the task of forming a structure having over 100,000 electrically discrete highly insulated metal plugs extending through the target and insulated from a signal screen embedded in the target being a tedious and diiiicult manufacturing problem. In addition, such electrodes, even though prepared with exceptional care, produce a spurious signal not representative of the electron or opitcal image, due to electrical or mechanical nonuniformities over the target surface.

In my above mentioned copending application I disclosed a tube incorporating and a method of manufacturing electrodes equivalent to double-sided electrodes, which tube I claimed in said application, and I will here set forth and claim my previously described method of manufacture as well as an improved modification of the method. The article produced by the improved method will be described below in general terms. A fuller and more detailed description of my new electrode structure is set forth in my application filed concurrently herewith and patented July 2, 1946, No. 2,403,239.

It is an object of my invention to provide electrode structure for a television transmitting tube having higher sensitivity and lower distortion than tubes constructed heretofore as well as a method whereby such structure may be manufactured with ease and at low cost. It is a further object of my invention to provide methods of manufacturing television transmitting tube electrodes suitable for use with low velocity electron beam scanning which are capable of exceptionally high sensitivity with substantial elimination of all spurious signal, and it is a still furtherobject to provide such a tube and target structure therefor which is easy to manufacture.

In accordance with my invention I provide an imperforate homogeneous target of electrically semi-conducting or semi-insulating material which is very thin in comparison with the electron beam diameter or picture element size, and a signal screen electrode very closely adjacent or adjoining thereto. Further, in accordance with my invention I provide a method of manufacturing a target of vitreous material wherein the material is stretched taut over a support member which in one modification may be the signal screen electrode. These and other objects, features, and advantages of my invention will be apparent when taken in connection with the following description and accompanying drawing in which:

Figs. 1 and 2 are views of portions of the electrode structure;

Fig. 3 is a greatly enlarged sectional View of a portion of a target structure and associated signal screen made in accordance with an improved modification of my method of manufacture.

Fig. 4. is a section illustrating a modified meth- 0d of applying the target film.

Figs. 5 and 6 show details relative to the mounting of the glass film on the screen.

Fig. '7 illustrates a modification.

The target of my invention comprises a very thin imperforate target sheet IQ of homogeneous electrically semiconducting material, the properties of which have been set forth in my said original application, and reference is made to it for a disclosure thereof as Well as for the manner of utilizing the target. Closely associated with the target sheet IQ of semiconducting material and preferably adjacent or adjoining one surface of the sheet I provide a signal screen 20 coextensive with and very closely spaced with respect to the surface of the semiconducting sheet. The enlarged fragmentary section of Fig. 3 shows a target sheet 19' and a type of adjoining signal creen 20' more particularly described and claimed in my currently filed application when made in accordance with my improved method.

The target sheet I!) or H! as indicated above is of semiconducting material and I have found that the specific resistance of the material used should lie within the range of 10 to 10 ohm-centimeters. Preferably an exceptionally thin homogeneous sheet of vitreous material such as glass having the desired specific resistance and being smooth and of uniform thickness may be used. I have found that a thin sheet of glass having a specific resistance of approximately 5xl0 ohmplaced in a furnace very carefully and c'arefully heated until the glass film is in a semi-molten state, whereupon by surface-tension the bulging sheet of glass contracts to form a sub'stantially plane surface. During the heating the thin glass film folds down against the edges of the ring 21 as shown in Fig. 2 as the film preferablyoverlaps' the ring. The film may be inverted and-placed on top of the frame with the bulge upwards when placedin the furnace as-shown in dotted lines in Figures. Thewireinesh'signalscreen 2n is'then assembled in very close ,proiiimity' to the sheet I 9 asb'y a-slip 'ring construction shown in Fig. 'Z 'ivIierein the'metaI ring 22 supports the signal screen zn. I'ha'vef made such, assemblies wherein the tln'cknessof the'shee't of glass is less than c. 660 ",whichis less'than'o'ne-foiirth the thickness -oiniica "usiially 'used for high resistance tl l cs df mosaic electrodes.

While an electrode 'fma'def in "accordance with the method described above is satisfactory when 'the'electrode area" is relatively small, 'I' have found thata "gia'ss'shees "mounted near a fine mesh screen cannot readily be spaced'close enough or uniformly 'enou'ghtopr'ovide good insulation and signal sungthtver relatively large target areas.

Thus, it is especially difficult to maintain uniform spacing between thesignal screen 'Zl'l "and the sheet l9 as shown in Figure 2 whnthe target is to-be of "runway large area.

Inaeecr'daiice with a"'mo'dification of the above describes fnbodiniehtbf 'my invention I utilize the prihc'iplesbf shrinking a thinshe'et of glass by surrace'tensibn over a supporting member or foundation and so perform theprocessthat' very small areas ortne'g'lasssneeeare in'sontaet with the supporting member which in this modification may be very small elemental areas of the signal sereh 2'0. To p'rovfde uniform spacing when utilising asig'nai screenfof' the type shown in '2, it is desirable to'ioll the screen so'that the wires of the'wdof and'the wires-of the warn are substantially coplanar. theimproved modification, however, and especially where target 'electrodesfor use in tubes of the type shown in mysaid brig inal'appliation of relatively large areas 'aretdbe iitilize'd,I have found that it is desirable to utilize woven wire -mesh which 'is' not rolledorat least is not'rolled to any great degree so that'alternate points of the 'wire of the crossed me'sli'wires'extend above the normal plane of the signal screen. Thus the crimp promontories' of the crossed'wires extend above the main body of thescre'en. It is also desirable in this "modification of my invention to provide a thin coating of vitreousenamel or other vitreous material on the" side of the signal screen adjoining the thin "glass' sheet is for the purpose of bonding the screen'tothe sheet over the raised promontory "areasfandto insulate the'screen from'the thin f'glas's sheet. While athin glass sheet 'ma'ybe fusedf-to an enameled rolled "fine mesh screen, thethin-glass tendsto "be drawn into theapertures and'become'thickened in the apertures if the temperature'of'the firing exceeds a predetermined maximum. In addition the electrical 4 properties of such a structure are unsatisfactory especially where the wire screen is greater than 150 mesh per inch. While such an assembly is satisfactory when utilizing a relatively coarse mesh screen, such as less than 150 mesh per inch,

the structure is far from satisfactory when utilizing a fin mesh screen, such as from 150 to 250 mesh per inch. I have found that, when "utilizing my method of shrinking the glass by :surfa'cetension, the thin sheet of glass is substantially tangent to the surface of the woven wire mesh at the crimp promcntories. Thus, it

is'impo'rtant that 'the screen following weaving should notbe rolled to the extent of flattening the normal crimp that appears in the wires, in fact,"-the existariceof the crimp permits the glass to touch only at the crimp promontories and still be spaced on the average far enough from the signal screen to provide' the desired target capacitance as described in my said original application.

Follbwing'the weaving of the wire'me'sh'screen to form thesi'gnalsereen'za' 'as 'shown inFig. 3, the mesh is preferably immersed in'a'high' current electrolytic etching solution to obtain a high poiish on the mesh and to increase the interstitial areas between the crn'ssedwires. For example, a 230'm'esh' per inch screenma'y 'beetched to about 65% "light transmission, the etching solution being chosenwith respect to the'particular metal of which the signal screen is made. .Ihave used stainless steelwire mesh inasmuch asits coefiident of expansion is sufficiently'closeto that of the glass, although an even closer match between the coeflicients of "expansion may be 'desirableto eliminate an erirlknng or the glass upon cooling. I

The'etche'd screen issupported'from its two opposite sides for example on atripo'd'and "held by stutableciampsfl'a' as shown" in Fig. 5, so as to fdiln a'eylindrical'ShaJDe lin'dftheweig'ht Of a 'cyliridricalform such ashe'avyglass tubing 44 and the screen is then fire'd in air an oven, while supporting the g'lass' tubing, at .a relatively low" temperature for a sufficient length of time to armament-coatin ofoiridean'd to'fix the shape of'tne' screen. v

'Following the firing operation the screen is enameled atleast :on the -co'nvex 'side which, as

later described, is'the side nextto the glass diaphragm with a low'temperature'high resist ance enamel. This enameling operation-may be performed by I a spraying method :wherein the enamel is suspended in-a finely divided state in a suitable suspending agent which is sprayed over either one or looth-sides/of the screen. The thickness of the enamelshould-beinsufl'icient to reduc the light transmission of the-screen below 50% and likewise should be of insufficient thickness to destroy the crimpwontour of the Woven wires.

I have-found thickness of approximately .0001 to 0002 inch suiiicientto provide a goodbond between the metal andsglasssheet wi-thout destroying the crimp contour. One particularenamel which I have found satisfactory is that of the followingcomposition by weight.

-Percent Following th application ofthe enamel to the wire mesh, the wire mesh is fired at a suificient temperature to vitrify the enamelparticles and to form a smooth coating over the wires of the screen while still retaining the crimpedicharacter of the wire mesh. In this manner the crimp promontories coated with the vitrified enamel extend above the surface of the screen. A section of thin glass which approximates the cylindrical contour of the formed screen is taken preferably from a bubble blown from glass chosen to have a softening point somewhat above that of the enamel. This glass is then sealed to the insvlated crimp promontories of the screen. For use with wire mesh screens coated with the above enamel a glass of the followingcomposition is particularly satisfactory wherein the percentages are by weight:

Percent N azO 37 L120 1 B203 16 A1203 2 S102 44 The thin glass section 45 from the cylindrical blown bubble is laid over the enameled surface of the wire mesh screen supported on the cylindrical glass tubing Fig. 6 and fired in an oven at or slightly above the softening point of the thin glass, whereupon the surface tension of the thin glass draws it taut over the surface of the enameled screen. Since the enamel has a lower temperature softening point than that of the thin glass, it tends to adhere to the thin glass sheet at the points of contact. such as at the crimped promontories. The promontories tend to prevent the glass from drawing into the intersticies of the mesh and the firing is controlled over such a short period of time or at such a low temperature to prevent such action. Firing for too long a period of time or at too high a temperature permits the thin glass to sink into the holes of the screen. The temperature and time control prevents this unsuitable result. Following the sealing of the thin glass sheet to the glass promontories, the assembly is lifted from the section oftubing and flattened by firing it on a plane surface at a relatively low temperature insufiicient to replastize the thin glass sheet. Due to the extreme thinness of the film and screen and the heating, the assembly sinks down fiat upon the plane surface, whereupon after cooling the assembly remains fiat and is ready for use.

Referring to Fig. 3 which shows a structure made in accordance with this latter method the wires 40 are covered with the thin film of enamel ll in such a manner that the crimped promontories 42 are not destroyed. It will be observed from the figure that the thin glass sheet I9 is in contact with the enamel screen only at the crimped promontories 42 and that the enamel 4! has become sealed to the glass sheet. In this manner uniform spacing between the sheet I 9' and the wires of the mesh is maintained even over a relatively large area. While I have described an assembly wherein the wire mesh screen may be coated over its entire exposed area with a thin film of enamel, the use of such an insulated screen requires the use of an auxiliary electrode to collect secondary electron emission. In the absence of a longitudinal magnetic field, such as developed by the coil 4'! (Fig; 7) the accelerating electrode coating 46 may serve to collect secondary electrons developed by impinging primary electrons from the photocathode 49. If, however, the longitudinal magnetic field produced by coil 41 is utilized, I have found it de-- auxiliary electrode the signal screen 20' of Fig. 3'

may be enameled on only one side; that is, the side adjoining the thin glass sheet. The secondary electrons may then be collected by the screen at the unenameled side.

lWhile I have described the use of an intermediate thin coating of enamel upon the wire mesh of insufficient thickness to destroy the crimped promontories, the thin glass sheet may be sealed directly to these crimped promontories without the use of such an intermediate enamel film. For example, the above steps may be performed with the exception of those relating to theapplication and firing of the enamel, thereupon the thin glass upon firing and shrinking by sur-. face tension becomes attached directly to the crimped promontories which are of exceedingly small area in comparison to the elemental picture area. Such electrode structure, however, is

f more diflicult to utilize, inasmuch as the glass of the sheet I9 is of relatively low electrical resistance and it is therefore necessary to provide substantially uniform electron beam velocity over the entire scanned areas.

The target, whether of the form shown in Fig. 2 or that shown in Fig. 3, is supported in the tube as described in connection with my said original application and in operation high velocity photo electrons representative of an electron image of the picture to be transmitted are focused on the rear surface of the thin target sheet to form a positive electrostatic image on this rear surface by secondary electron emission. Other methods of forming a positive electrostatic image on the target sheet may be utilized, such as by scanning the rear surface of the sheet with a modulated high velocity electron beam or by providing a discontinuous coating of caesium or other light sensitive material on the rear surface of the sheet which, by emission of photo-electrons, forms an electrostatic image consisting of positive charges representative of the optical image to be transmitted. Thus in the latter case the light from the object may be focused directly on the rear surface of the target sheet 19.

An electrode structure made in accordance with my invention may be distinguished from the conventional double-sided type of mosaic such as the type described in my U. S. Patent 2,213,173 in that I provide no conductive plugs extending through and insulated from a wire mesh screen embedded in high resistance insulation such as vitreous enamel. In this type of construction it is necessary to provide exceptionally high resistance between theplugs and the embedded screen for the express purpose of preventing electrical leakage between the plugs and the screen. In my improved television transmitting tube I avoid the difficulties attendant upon the prior art structure by wholly eliminating the embedded screen. I have found it disadvantageous to provide any good metal contact between the semiconducting target sheet l9, which must be homogeneous, and any other electrode in the tube such as the signal screen 20'. As indicated above, this desirable feature is obtained either by supporting the signal screen 20 out of contact with the glass sheet I9 or by providing a thin coating of high resistance'enamel between the screen and glass sheet. The signal screen agazzsyaoo 2G must therefore be andis substantially l'atedi from the-sheet I29." so that. electrica-ii comtactsbetween the" signal screens andithe sheets t8! are-a. minimum. invention is; further distinguishedl over. the. rior art which utilizes. a mica foundation. for a. mosaic electrode on anyv other insulation; having high. electricalaresistancc such: as mica which. is. ofthe order: of ohm':-

centimeters; My invention should. therefore: not be confused with tubes usingitliese types of elece trodes. In; fact, I have: attempted to operate tubes of this: type: wherein. the target; consisted ofi a mica: foundation scanned-on; one: side by 2: low: velocity electron: beam, the electrostatic imager being formed on. the opposite: side: and? have found. that such .tu'oes will not "operate with: 9;.10WVB1OCity electron. beam because after a.f-ew' scamsionsi of the: beam;v elemental areasof; the: target: become" charged up to" .thesecondary elea tron: collector potential thereby. preventing, any; further operation. With myrnew: andFimprQiie'd tubmhuwevenlhave obtainedif-iomiii to lKJ-times-- the: sensitivity usually obtained in tubesof the Orthiconf" type as. described. in; U; S... Batents 2,2l.=3;lfl4"-61 which, in. turn; are from: 10 tot '50. times more sensitive. than the conventional Iconoscope which: utilizes a high velocity elemtron beam. Consequently; the televi'sionxtransmitting tubes utilizing electrodes: made: he amcordancewith my. invention: when'utilizedimconjunction witlnlowsvelocit electron; beam scanning; havexpnoduced. sensitivitiesof the order of; to SKllltimes.the sensitivityof:the well-knownlconm scope type of. tube.

While: I have indicated the preferred. embodiments: of: my; invention and have indicated the specific. application: as directed to: the production of: electrodes for cathode ray; television; transmit.- ting. tubes, it will be' apparent. that my: invent-10m isby: no: means: limited to the purpose oft television transmission and thatmany variationsrman beimade in the particular method. and: structure disclosed. without departingfrom the scopeioff the invention as-setforth in theappended claims;

I claim:

1=. The-method=l of manufacturingzatarget: elece trodefor a: television transmitting tube. comprise inggf'orming:v aibubble -otvitreous.-material, cutting; azsection of. film therefrom; supporting'saidi film on a horizontal frame", heating: said film: and framerwhile said frame'istin a. horiZonta-ltposition to a: temperature sufficient. to render said film semiplastic, and maintaining said temperature untilithe surface tension of saidsvitreous material causes-said film to contr actand tightem onl said frame.-

2-. Themethod: of manufacturing:aatarget-elec trade for at television transmitting; tube; which comprises placing a thin non-planar fiimofivitrez. ous. material over a. horizontal support having areas to which said filmmay be'sealed;.and1heat; ing: said film and support to a: temperature: at which it becomesattachedto said horizontalsup portend. at which. the. film material is plastic and: the surface. tension of the film material causes the film-to flatten.

3. The. method of manufacturing--21. target. electrode; for a. television transmitting tube com.- prising heating a thin filmof, vitreous material iii-contact witha horizontal supporting member to. cause saidvitreous materiale t become.v at? tachedito said.member and continuing theaheat. ing: until. said: film shrinksby surface tension: to become taut over said member.

e; The-meth'cd of manufacturinmatarset-eiec 1- surface tension to become; taut.

5. 'l iiemethod of manufacturing a target elec trode adapted'lto be: scanned by: an electronbeain comprising heating: an apertured" crimped wire mesh screen in horizontal contact with a: thin vitrecusz-materiat film! until the film. seals: to and becomes taut over the said screen and -discon tinuing-isaid heating before the film becomes sufficiently soft to sink into and become thickenedin theapertures of'said screen.

6'. The method of. forming. a target electrode adapted to be scanned by an electron beam comprising supporting. a non-planar thin glass filmin contactwith an insulated woven mesh foundation;. said? foundation having regularly spaced promontorias over one side thereof and being hcri'zonta:lly-disposed; heating the film andicontinuing said heating until said film: becomes sealed at said promontories substantially tangent tb 'tlie horizontal surface of said foundation.

7". The method of" forming; a target electrode adapted. to be scanned" by an -electron'lrlea'micomprising; coating one side of a woven" wire mesh screen with a thin fil'mof eIectricaH insuIatin'g material leaving the apertures offsaid'screen unfilled; supporting a thin glass film on? said. insulated screen andheating said screen until the film adheres to small elemental? areasoverthe surface: o'fsailiiihsulated screen and becomes taut by surf ace-tension.

8': The method of manufacturing atargett electrode for television transmitting? tubes comprisin}; supporting: a; thin glass film: on a horizontal support member; heating said film" an'd'sa'id member'until said film becomessemi-plastic and becomessealedto said member. andcontinuingsai'd heating? untiltlie' film shrinks by surface tension andbeoomes'tautrandsubstantiallyplanebetween separated points? over." the surface? of saidsupport member.

'9. The method of attach-ingja. thinglass film toa Wire screen, comprising, forming the screen into a cyiindri'cairsurfa'ce; placing. it on; top ofa cylinder with the convex. surface of the screen extendingupwards, placing the thin glass .film' on topof the screen; heating the screeni'until. the glass filmi conforms to the. shape of the screen, fusestheretoandbecomes taut'bysurface tension, cooling the" filmand screen and reheating, on a fiat support until the screen and attachedffilm flatten out-"substantiallyt'o atplane.

10: The methodcf" attaching a glass film to a wire screen, comprising: formingthe screen into avcylindrical surface; enameling'the screen, supporting the" screen with convex: surface. ext'ending upwards, placing thethi'n glass film on top of the screen, heating the filmand screen until the glass film conforms to the-'shape-of the screen; fuses to. the enameled crc'ss over points of. the; of the. screen: and: becomes taut" by surface: tension "cooling: the-film and-"scream and reheating om aaflat' support until. the screen and attached film: fiattenv out'ssubs't'antia'llyto. apla-ne.

1-1.. Ther'method of?mamifacturing-xaztareetelem trode. for awtelevislon: transmitting tube: comprise jinaplacinaa section of aeIassbubblemnwe-frame and heating the glass section and frame to a. temperature at which the glass is plastic, becomes attached to said frame and draws taut thereacross by Surface tension and cooling the frame and attached film.

ALBERT ROSE.

REFERENCES CITED The following referen'ces are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 844,767 Arbogast Feb. 19, 1907 1,395,963 Kuppers Nov. 1, 1921 Number Number 

